The present invention relates to a method of determining the point at which a string of tubing has become stuck within a wellbore. The present invention also relates to a string of tubing for performing a primary operation in a wellbore, which includes equipment to facilitate determination of the point at which the tubing has become stuck, should such occur during translation of the tubing relative to the wellbore.
In the oil and gas exploration and production industry, wellbore fluids comprising oil and/or gas are recovered to surface through a wellbore which is drilled from surface. The wellbore is conventionally drilled using a string of tubing known as a drill string, which includes a drilling assembly that terminates in a drill bit. Drilling fluid known as drilling ‘mud’ is passed down the string of tubing to the bit, to perform functions including cooling the bit and carrying drill cuttings back to surface along the annulus defined between the wellbore wall and the drill string.
Following drilling, the well construction procedure requires that the wellbore be lined with metal wellbore-lining tubing, which is known in the industry as ‘casing’. The casing serves numerous purposes, including: supporting the drilled rock formations; preventing undesired ingress/egress of fluid; and providing a pathway through which further tubing and downhole tools can pass. The casing comprises sections of tubing which are coupled together end-to-end. Typically, the wellbore is drilled to a first depth and a casing of a first diameter installed in the drilled wellbore. The casing extends along the length of the drilled wellbore to surface, where it terminates in a wellhead assembly. The casing is sealed in place by pumping ‘cement’ down the casing, which flows out of the bottom of the casing and along the annulus.
Following appropriate testing, the wellbore is normally extended to a second depth, by drilling a smaller diameter extension of the wellbore through a cement plug at the bottom of the first, larger diameter wellbore section. A smaller diameter second casing is then installed in the extended portion of the wellbore, extending up through the first casing to the wellhead. The second casing is then also cemented in place. This process is repeated as necessary, until the wellbore has been extended to a desired depth, from which access to a rock formation containing hydrocarbons (oil and/or gas) can be achieved. Frequently, a wellbore-lining tubing is located in the wellbore which does not extend to the wellhead, but is tied into and suspended (or ‘hung’) from the preceding casing section. This tubing is typically referred to in the industry as a ‘liner’. The liner is similarly cemented in place within the drilled wellbore. When the casing/liner has been installed and cemented, the well is ‘completed’ so that well fluids can be recovered, typically by installing a string of production tubing extending to surface.
It is known that the various different types of tubing run into a wellbore can become stuck. For example, a drill pipe can become stuck during the operation to drill and extend the wellbore. Wellbore-lining tubing (casing, liner) can become stuck during deployment into the wellbore and prior to cementing in place. Primary reasons for the tubing becoming stuck include: cave-in of the drilled rock formation; and a condition known as ‘differential sticking’. Differential sticking typically occurs when the pressure of the formation being drilled is significantly lower than the wellbore pressure, resulting in a high-contact force being imparted on the tubing, against the wall of the drilled formation. Differential sticking can be a particular problem in deviated wellbores.
The recovery of a tubing which has become stuck in a wellbore can be extremely challenging. Initial efforts to retrieve the tubing typically involve ‘jarring’ the tubing, by imparting a short duration large axial force on the tubing, and/or by rotating the tubing. However, often this does not work, and so a range of different techniques and equipment have been developed for recovering stuck tubing.
The main techniques which have been developed centre around locating the point at which the tubing is stuck, and then imparting a localised axial and/or rotary force on a joint of the tubing which is located as close as possible to that point. Following release of the joint, the portion of tubing above the joint can be retrieved to surface, and a specialized tool know as a ‘fishing tool’ run in, to impart a large pull force on the remaining portion of tubing to retrieve it.